The present invention relates generally to the manufacture of mineral fibers, such as glass fibers, and relates more particularly to the scrubbing of effluent gases arising during the production of mineral fiber blankets, mats or boards.
The manufacture of mineral fiber insulation products conventionally includes as one of a series of operations, the step of forming a blanket or mat of fibers which then undergoes further treatment and transformation into various types of products. During the initial formation of the blanket, as well as during the subsequent treatment and transformation steps, substantial quantities of effluent gases are developed which require treatment because of entrained polluting elements.
In the formation of the fiber blanket, the newly formed fibers are typically carried by gas currents onto a foraminous conveyor moving transversely to the direction of gas and fiber flow, the gas passing through the conveyor while the fibers accumulate on the conveyor surface in the form of a blanket of substantial thickness. In the conventional process, a binder spray is directed onto the fibers prior to their collection on the conveyor.
Since the gases passing through the conveyor contain binder either in vapor or droplet form as well as some fibers and mineral particles which have passed through the conveyor, it is necessary to provide means for scrubbing these gases before their recycling or release into the atmosphere, especially since the binder formulation can be quite toxic as can the degradation products which are formed when the binder comes in contact with the high temperature fibers.
Although the gases described resulting from the fiber blanket laydown are a principal source of effluent gases requiring scrubbing in a mineral fiber production system, other operations also produce gaseous effluents which require treatment. Principally, these include the treatment of the blanket to cure the binder, which treatment stage normally includes a curing oven possibly followed by a cooling treatment, and subsequent transformation operations wherein the blanket is for example combined with other elements to form a laminated structure, treated with additional compositions, combined with facing sheets, cut into predetermined lengths, etc. Many of these subsequent treatment and transformation operations are sources of effluent gases which require scrubbing, particularly those involving heating or cooling flows of air, spraying of compositions, or cutting operations which might typically include sawing, slicing or severing by means of high pressure air blasts.
The present invention has accordingly been developed to treat the effluent gases arising from various types of mineral fiber production operations. The effluent gases, whether resulting from the fiber blanket formation region or subsequent treatment or transformation steps, usually includes a combination of particulate solids such as unfiberized mineral fragments, broken or cut fibers, and in addition chemical compositions such as the binder spray which may be either in gaseous or droplet form. This combination of binder droplets or vapors and fibers or fiber fragments not only create a need for a thorough scrubbing of the gases but furthermore creates a problem within the chambers and ducts through which the effluent gases pass. The binder tends to form adhesive deposits which retain the entrained fibers or fragments resulting in a buildup of deposits on the chamber and duct walls. Such buildup of deposits is quite rapid and requires the periodic stoppage of the process to permit their removal. Not only do such stoppages result in lost production of fibrous material, but the removal of the deposits is itself a laborious and costly operation. The occurrence of such deposits is thus a significant factor in terms of production costs of fiber.
The conventional approach to treating the effluent gases has been a passage of the gases through atomized water sprays for the purpose of cooling and initially removing a large amount of the polluting elements. In particular, an effort has been made to remove as much binder as possible by this water atomization to prevent the fouling of the chamber and duct walls. Difficulties are however encountered in effectively implementing the water atomization.
A first difficulty results from the extremely large quantity of gas circulating through the typical installation and, consequently, with the dimensions of the installation within which the scrubbing operations must be conducted. In U.S. Pat. No. 4,111,672, some values of gas volumes are indicated, characteristic of different modes of fiber production. These values are on the order of 0.1.times.10.sup.6 to 1.times.10.sup.6 m.sup.3 /hr. of effluent gas for the operations resulting in the formation of the fiber blanket. As described below, it is difficult to obtain a fine, homogeneous dispersion of the atomized water on such large volumes while using conventional atomizing techniques.
A second difficulty in implementing effluent gas scrubbing by water atomization arises from the need to prevent the formation of deposits on the path of the gases as soon as the gases have passed into the gas evacuating chamber or duct. With respect to the gases flowing from the fiber blanket forming area, the deposits must be prevented immediately beneath the conveyor. Otherwise the deposits will modify the passage section and consequently the flow of the gases across the blanket being formed. Such modification would interfere with the homogeneity of the fiber blanket. The problem of deposit prevention is further increased by the constraint that the water projected during the scrubbing operation cannot be permitted to reach the fiber blanket.
A third difficulty arises from the fact that the water used, which becomes saturated with polluting elements, cannot be discarded. It is therefore conventional to recycle the water after it has been cleansed of at least a portion of the entrained polluting elements. In order for the cost to be acceptable, the water cleansing operation must be relatively simple, for example, a summary filtration or a similar operation. Following such treatment, the recycled water ordinarily still contains a substantial quantity of materials in suspension as well as stable or unstable dissolved products.
In the conventional scrubbing process used with mineral fiber production systems, the atomization of the water is effected by passing the water under pressure through nozzles having relatively small dimensions. This method, however, suffers several serious disadvantages. First, the quantity of water distributed by each atomizing nozzle and the area effectively treated by the atomization from each nozzle are necessarily quite limited because of the small nozzle dimensions. Although it is possible to increase the number of nozzles, it is nevertheless difficult to attain a complete continuity and good homogeneity of the layer of water droplets throughout the entire area tranversed by the gases. In practice, even with a large number of nozzles of this type, it is not possible to effectively treat all of the gas flow and, consequently to prevent the formation of deposits on the walls of the chambers or ducts.
Secondly, because of the nozzle dimensions, frequent cloggings of the nozzles are experienced, especially as the recycled water becomes more saturated with polluting elements. Therefore even a good arrangement of the nozzles in the atomizing area cannot guarantee a continually homogeneous atomization. Furthermore, the nozzle cloggings require frequent interruptions of the process for reconditioning of the nozzles.
Thirdly, the nature of the trajectory of the atomized water from the conventional nozzle makes it difficult to position the nozzles close to the fiber blanket receiving conveyor because of the danger of the water reaching the fiber blanket. The conventional nozzle array is accordingly located at some distance from the conveyor and directed away from the conveyor, thus providing an opportunity for deposits to develop on the chamber walls immediately adjacent the conveyor.
In an effort to overcome these difficulties, the conventional nozzles were replaced by an apparatus in which the water dispersion is obtained by projection onto a concave curved element. A water jet directed onto such element forms a liquid layer which widens and bursts into a multitude of droplets.
There are several drawbacks to such a method of atomization. A first difficulty is that very fine droplets can only be produced with relatively low output. Secondly, there is a rapid wearing of the water dispersion element because of the abrasive particles in the water. Deterioration of the polished surface of the dispersion elements take place rapidly and within a few days, the erosion-corrosion of the elements causes the deformation thereof and causes the elements to become less efficient. It is usually necessary to replace the elements after two weeks of continuous operation.
In view of the foregoing difficulties with existing systems for scrubbing effluent gases from mineral fiber production facilities, the following objectives of the invention can be appreciated.
A first object of the present invention is to provide a scrubbing of the effluent gases arising from mineral fiber production which prevents the formation of deposits on the walls of the chambers and ducts through which the gases pass.
Another object of the invention is to provide, in connection with the gases passing through the fiber receiving conveyor, an arrangement for scrubbing the gases immediately beneath the conveyor without the atomizing water reaching the conveyor in order to maintain the cleanliness of the walls in this region.
Still another object of the invention is to provide an extremely homogeneous water dispersion within the chambers and ducts through which the effluent gases flow, even when the chambers and ducts are of large size.
Another object of the invention is to provide a scrubbing of gaseous effluents from mineral fiber production installations which provides a good efficiency of removal of the polluting products carried by the effluent gases.
Still another object of the invention is to provide means for producing atomization of water which is capable of operation even with water containing abrasive particles during prolonged usage without effecting operation.
A still further object of the invention is to provide means for atomization of water which is operative even if accidentally saturated with relatively voluminous corpuscles without blockage of the atomizing means.
Still another object of the invention is to provide a scrubbing means as described for effluent gases which may be utilized to scrub effluent gases arising from all of the steps of the production process.